Disturbed peace in the West

Gratian governed the Western Roman Empire with energy and success for some years, but he gradually sank into indolence. He is considered to have become a figurehead while Frankish general Merobaudes and bishop Ambrose of Milan jointly acted as the power behind the throne. Gratian lost favor with factions of the Roman Senate by prohibiting traditional paganism at Rome and relinquishing his title of Pontifex Maximus. The senior Augustus also became unpopular with his own Roman troops because of his close association with so-called barbarians. He reportedly recruited Alans to his personal service and adopted the guise of a Scythian warrior for public appearances.

Meanwhile Gratian, Valentinian II and Theodosius were joined by a fourth Augustus. Theodosius proclaimed his oldest son Arcadius an Augustus in January 383 in an obvious attempt to secure succession. The boy was still only five or six years old and held no actual authority. Nevertheless he was recognised as a co-ruler by all three Augusti.

The increasing unpopularity of Gratian would cause the four Augusti problems later that same year. Hispanic Celt general Magnus Maximus, stationed in Roman Britain, was proclaimed Augustus by his troops in 383 and rebelling against Gratian he invaded Gaul. Gratian fled from Lutetia (Paris) to Lugdunum (Lyon), where he was assassinated on 25 August 383 at the age of 25.

Maximus was a firm believer of the Nicene Creed and introduced state persecution on charges of heresy, which brought him into conflict with Pope Siricius who argued that the Augustus had no authority over church matters. But he was an Emperor with popular support, as is attested in Romano-British tradition, where he gained a place in the Mabinogion, compiled about a thousand years after his death.

Following Gratian's death, Maximus had to deal with Valentinian II, at the time only twelve years old, as the senior Augustus. The first few years the Alps would serve as the borders between the respective territories of the two rival Western Roman Emperors. Maximus controlled Britain, Gaul, Hispania and Africa. He chose Augusta Treverorum (Trier) as his capital.

Maximus soon entered negotiations with Valentinian II and Theodosius, attempting to gain their official recognition. By 384, negotiations were unfruitful and Maximus tried to press the matter by settling succession as only a legitimate Emperor could do: proclaiming his own infant son Flavius Victor an Augustus. The end of the year found the Empire having five Augusti (Valentinian II, Theodosius I, Arcadius, Magnus Maximus and Flavius Victor) with relations between them yet to be determined.

Theodosius was left a widower in 385, following the sudden death of Aelia Flaccilla, his Augusta. He was remarried, to the sister of Valentinean II, Galla, and the marriage secured closer relations between the two legitimate Augusti.

In 386 Maximus and Victor finally received official recognition by Theodosius but not by Valentinian. In 387, Maximus apparently decided to rid himself of his Italian rival. He crossed the Alps into the valley of the Po and threatened Milan. Valentinian and his mother fled to Thessaloniki from where they sought the support of Theodosius. Theodosius indeed campaigned west in 388 and was victorious against Maximus. Maximus himself was captured and executed in Aquileia on 28 July 388. Magister militum Arbogast was sent to Trier with orders to also kill Flavius Victor. Theodosius restored Valentinian to power and through his influence had him converted to Orthodox Catholicism. Theodosius continued supporting Valentinian and protecting him from a variety of usurpations.
Final partition of the Empire

In 392 Valentinian II was murdered in Vienne. Arbogast arranged for the appointment of Eugenius as emperor. However, the eastern emperor Theodosius I refused to recognise Eugenius as emperor and invaded the West, defeating and killing Arbogast and Eugenius at the Battle of the Frigidus. He thus reunited the entire Roman Empire under his rule.

Theodosius had two sons and a daughter, Pulcheria, from his first wife, Aelia Flacilla. His daughter and wife died in 385. By his second wife, Galla, he had a daughter, Galla Placidia, the mother of Valentinian III, who would be Emperor of the West.

Theodosius was the last Emperor who ruled over the whole Empire. After his death in 395, he gave the two halves of the Empire to his two sons Arcadius and Honorius; Arcadius became ruler in the East, with his capital in Constantinople, and Honorius became ruler in the West, with his capital in Milan and later Ravenna. The Roman state would continue to have two different emperors with different seats of power throughout the 5th century, though the Eastern Romans considered themselves Roman in full. Latin was used in official writings as much as, if not more than, Greek. The two halves were nominally, culturally and historically, if not politically, the same state.